The invention relates to an anchor comprising an expanding sleeve and an expander plug; the sleeve comprises an expanding skirt with axial slots and, between the slots, expanding legs which between them form a conical axial bore which converges towards the free ends of the legs; the expander plug is also conical, with the same cone angle as the conical bore of the expanding skirt in which it is fitted; through a wedge effect, brought about by the expander plug being driven into the bore of the expanding skirt, the legs pivot and the skirt expands to anchor itself in the wall of the material which takes it.
Such anchors are well suited to materials of the concrete type. In the case of soft material, no problems arise. By contrast, in the case of hard material and because of the firmness of the legs and of the material, it is no longer possible for the plug to be driven far enough into the skirt and its expansion is not great enough to provide adequate anchorage.
The invention sets out to solve this problem.
To this end, the invention relates to an anchor for hard material having an expanding sleeve with an interior bore and an expander plug which are conical, with roughly the same cone angle, and designed so as, through the wedge effect caused by the plug being driven into the sleeve, to cause it to expand, the anchor being characterized in that it comprises means designed, during expansion, to shorten the axial expansion length of the expanding sleeve.
Thus, if the sleeve is functionally shortened, the plug can be driven in more fully and can correctly expand the functional axial portion of the sleeve engaged by the plug.
As a preference, the means of shortening the sleeve are designed to be able to drive the plug beyond the expansion end of the sleeve.
Advantageously, the shortening means are designed to shorten the expansion length of the sleeve by radially compressing an axial base portion of the sleeve.
In the preferred embodiment of the anchor of the invention, the shortening means comprise an annular weakening groove formed at the periphery of the sleeve, beyond which the axial base portion of the sleeve has an interior bore with a cone angle which is at least inflected with respect to that of the plug.